During formation and operation of a wellbore, it may be desirable to measure at least one property within a subterranean formation through which the wellbore extends. For example, a high pH may be a precursor of scale build-up and a low pH may be a precursor to corrosion of wellbore equipment. Thus, the pH of a formation fluid is conventionally monitored to aid in reducing scale build-up and potential corrosion of the wellbore equipment.
Conventionally, the pH of the formation fluid is determined by obtaining a sample of the formation fluid and analyzing the sample in a laboratory. However, as the formation fluid is brought from formation conditions (e.g., high temperature high pressure conditions), acid gases and salts may come out of solution, irreversibly changing the pH of the sample. Thus the analyzed sample may not be an accurate representation of the pH of the formation fluid at formation conditions.
Other methods of determining a pH of formation fluids include introducing a dye (e.g., phenol red, methylene blue, and/or cresol red) into the formation and correlating the pH of the formation fluid to the color of the dye. However, such dyes may not be formulated to determine the pH of the formation fluid with a desired level of accuracy. For example, some dyes may only be sensitive within a narrow pH range, such as a pH range of about 3.0 pH units. In addition, the dyes may be chemically unstable under formation conditions. Further, a continuous pH measurement may not be obtained unless the dye is continuously injected into the subterranean formation.
Other properties of the subterranean formation (e.g., salinity, wettability of formation surfaces, flow paths through the subterranean formation, etc.) may be determined using one or more tracer compounds. For example, water tracers may be introduced into the subterranean formation to estimate flow patterns between wells during enhanced oil recovery processes, such as, for example, water flooding. Some tracers may include a fluorophore (i.e., a compound that can re-emit light upon light excitation) and a presence of the tracer may be determined by fluorescence spectroscopy. However, the fluorophore may include organic molecules and rare-earth complexes that are toxic and/or radioactive and may contaminate the subterranean formation (e.g., aquifers located in the subterranean formation). Further, fluorophores may decompose at downhole conditions and may be subject to photobleaching (i.e., the photochemical alteration of the fluorophore such that it becomes permanently unable to fluoresce) and photo blinking (i.e., fluorescence intermittency).